This invention relates generally to radio frequency amplifiers and, more particularly, to radio frequency amplifiers having a plurality of cascade coupled field effect transistors arranged to provide a distributed amplifier.
As is known in the art, radio frequency amplifiers configured as distributed amplifiers having a plurality of cascade interconnected field effect transistors have been suggested to provide amplification of radio frequency signals. One such amplifier has been discussed in an article entitled "GaAs FET Distributed Amplifier," by J. A. Archer, F. A. Petz and H. P. Weidlich, published June 1981, No. 13, p. 433 in Electronics Letters. In such amplifier, biasing of the transistors therein is provided by coupling a direct current (dc) source to the output terminal of the amplifier through an inductor, such inductor providing a high impedance to radio frequency (RF) energy fed to the output port to thereby isolate the dc source from the RF energy while allowing a direct current (dc) path from the dc bias voltage source to the transistors. With such arrangement, however, when used with radio frequency signals over a relatively wide band of frequencies, a relatively large inductor would be required to provide the desired isolation and such inductor is difficult to realize. Further, in order to provide an amplifier adapted to operate over a relatively wide band of radio frequencies, it is desirable to fabricate the amplifier as a monolithic integrated circuit; that is, with the transistors and their electrical interconnects being formed on a single substrate, such as a gallium arsenide substrate. Thus, the use of a relatively large inductor for the biasing circuit to provide RF energy isolation to the dc bias source also makes the use of such inductor impractical for monolithic integrated circuit fabrication. Further, in order to meet requirements of a relatively wide bandwidth, it is also necessary that the biasing circuit provide smooth and well-controlled impedance variations over the band of frequencies to thereby optimize the amplifier design characteristics over such operating bandwidth.
Another distributed amplifier is discussed in an article entitled "Transistorized Distributed Amplifier" by C. W. McMullen published in The Review of Scientific Instruments, Vol. 30, No. 12, Dec. 1959. Here a dc biasing network is shown for the cascaded transistors in such amplifier. The transistors, here bipolar transistors, have their collector electrodes coupled to a DC voltage source input terminal through a resistor included in such bias network. The dc voltage source input terminal is also coupled to ground through a capacitor which provides a low impedance path for the radio frequency energy to provide some degree of isolation between the dc bias voltage source and the RF signals. With such arrangement, however, direct current passes through the resistor thereby causing dc power dissipation.